Generated by SassMeister.com.

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

@function map-combine($source, $map) {
  @each $key, $value in $map {
    $new: ($key: $value);

    @if map-has-key($source, $key) {
      $current: map-get($source, $key);
      $new: ($key: append($current, $value, list-separator($current)));
    }
    
    $source: map-merge($source, $new);
  }
  
  @return $source;
}

$map-1: (
  'foo': 'bar',
  'baz': 'qux'
);

$map-2: (
  'foo': 'qux'
);

.foo {
  content: inspect(map-combine($map-1, $map-2));
}

SassMeister-output.css

.foo {
  content: ("foo": "bar" "qux", "baz": "qux");
}